This invention relates to switched mode electrical power conversion and more particularly to a low EMI semi-resonant inverter DC power supply.
Conventional switching inverters using pulse width modulation (PWM) for regulation are well known to provide significant energy and space savings in a variety of power supply applications. In a typical circuit, the primary of an inverter transformer is repetitively switched between positive and negative sides of an input voltage source for inducing an AC voltage at the transformer secondary. The AC voltage is rectified and filtered to obtain a DC output voltage. Regulation of the DC output voltage is obtained by varying the duration (pulse width) of fixed frequency switching control signals in response to changes in the DC output voltage.
A principal disadvantage of switched mode power supplies of the prior art is that high levels of electromagnetic interference (EMI) are associated with operation of switching devices in inverter circuits.
High levels of both conducted and radiated EMI, especially at radio frequencies (RF) can be detrimental to proper operation of equipment utilizing switched mode power supplies as well as other equipment located nearby. Increasingly stringent government regulations limiting electrical interference have resulted from increased interference to radio communications by a proliferation of incidental signals generated by electronic equipment. Electronic products marketed in the United States must meet Federal Communications Commission standards for RF emmissions under Parts 15 and 18 of the Federal Communications Act. In European countries which are in close geographical proximity and where the radio frequency spectrum is nearly saturated, particularly strict rules are in effect.
A relatively new solution to the EMI problem in switched mode supplies is to incorporate a series resonant circuit in the inverter, limiting the switching signals to a fixed pulse duration, and using frequency modulation (FM) of the pulse repetition rate for regulation instead of the conventional PWM at a fixed-frequency. Significantly reduced EMI results from switching the inverter when current in the switch is zero.
With reference to FIG. 1, a prior art series resonant FM power supply 10 includes a pulse generator 12 having a pair of transformer-coupled control outputs 14a and 14b, for driving a corresponding pair of power switches 16a and 16b. During a fixed first time period the control output 14a drives the power switch 16a to connect a transformer primary 18 through a series inductor 20 to a positive input voltage. During a fixed second time period equal in duration and immediately following the first time period, the control output 14b drives the power switch 16b to connect the series inductor 20 to a negative input voltage. The transformer primary 18 is coupled in series to the negative input voltage by a series resonant capacitor 22, the series resonant capacitor 22 being clamped between the positive and negative input voltages by a pair of primary rectifier diodes 24. An induced AC voltage at a transformer secondary 26, rectified by a pair of secondary diodes 28, produces a DC voltage across a filter capacitor 30. The DC voltage is fed through an additional low-pass filter comprising a filter inductor 32 and an output capacitor 34 to produce a DC output voltage.
The DC output voltage is fed back to a voltage controlled oscillator 36 for regulation by frequency modulation of the pulse generator 12. The series inductor 20, the transformer primary 18, and the series-resonant capacitor 22 operate as a resonant circuit to produce sinusoidal current pulses in the transformer primary 18. The sinusoidal current pulses, having frequency components mainly at the fundamental resonant frequency, account for a significant reduction in EMI.
The FM fundamental resonant power supplies of the prior art have several disadvantages. For example:
1. Limited dynamic range. The dynamic range of regulation is directly proportional to the relative frequency range of the pulse generator 12. The maximum frequency is limited by the switching characteristics of the pulse generator 12 and the power switches 16. The minimum frequency is limited by transient response and output filtering considerations. Thus regulation from no load to full load cannot be achieved without wasteful use of a dummy load and/or an auxillary shunt regulator.
2. High circulating currents. The resonant circuitry produces high peak current levels approximately 40 percent higher than in conventional power inverters. The power switches, transformer, series resonant inductor and capacitor must each carry this high current. Consequently, the power supply is wasteful of energy and more expensive to produce, requiring components with increased current and power ratings and means for dissipating the wasted energy.
3. Circuit complexity. The FM control circuits required for regulating the output voltage while maintaining resonance are more complex then conventional PWM control circuits, resulting in higher manufacturing costs.
4. Low power rating. The maximum output of the FM fundamental resonant power supplies is limited to about 100 watts due to the unavailability of suitable capacitors having low equivalent series resistances and high ripple current ratings for the resonant circuit and for filtering the output voltage.
Accordingly there is a need for an inverter power supply that produces low EMI, has high output power capacity and wide dynamic range, and is inexpensive to produce.